Exactly what molecule in a mammal's egg cell talks to sperm to let them wriggle inside has been difficult to identify.

In 2005, scientists discovered that sperm carry a protein called Izumo1 that gets them to fuse with an egg. Now, research on mice shows that it's the protein folate receptor 4, or Folr4, on the egg that plays the partner role to bring the reproductive cells together. The discovery, published April 16 in Nature, could lead to better contraceptives and fertility treatments. The team also suggests renaming the protein Juno after the Roman goddess of fertility and marriage.